The present invention relates generally to Fourier transform mass spectrometry and more particularly to the manipulation of data in a Fourier transform ion cyclotron resonance (FT/ICR) mass spectrometer.
Chemical applications of Fourier transform ion cyclotron mass spectrometry are fully described in the Accounts of Chemical Research, Vol. 20, page 316, Oct. 1985. Fourier transform spectrometry consists of acquisition of data points as a function of time, followed by discrete Fourier transform to yield the frequency domain spectrum. The major advantages of the Fourier Transform mass spectrometer (FT/ICR) over other mass spectrometer designs are its relatively simple mechanical construction and its use of sophisticated modern electronics and computer engineering, which allow ion manipulations, high mass resolution, large mass range, and simultaneous detection of all the ions.
An ion cyclotron relies on the use of a fixed magnetic field, B, to deflect an ion of charge, q, moving at a certain velocity, v, according to the Lorentz force, F=qv.times.B. For spatially uniform B, a moving ion of mass, m, will be bent into a circular path in a plane perpendicular to the magnetic field, with a natural angular frequency, .omega., [.omega.=qB/m]. Thus, if the magnetic field strength is known, measurements of the ion cyclotron frequency in eq 1 suffices (in principle) to determine the ionic mass-to-charge ratio, m/q. In other words, in a static magnetic field the mass-to-change ratio is uniquely determined by the ion-cyclotron frequency.
Until now, most of the previous FT/ICR systems have used relatively expensive minicomputers. In addition to the use of a minicomputer, specially dedicated fast hardware memory is needed. The fast hardware is necessary for computers which use dynamic random access memory having its own refresh cycle asynchonous to the execution of instructions. Also, the time necessary for data transfer instructions is generally slow thereby requiring extra hardware control circuitry.
In these FT/ICR systems, a general approach to improve the signal-to-noise ratio is to average the signals. This may be accomplished in two ways. The first is to perform fast Fourier transform (FFT) on each experiment and average them in the frequency domain. This does not require identical phase relationships between each signal subjected to FFT, but does require an enormous number of calculations.
The second method takes time-domain signals, averages a set and performs FFT on the average of the set. In the latter method, the phase relationship between each member of the set must be exactly the same. Fast hardware is necessary to control and monitor the ion-cyclotron events. Problems arise if timing is software controlled unless that computer and its programs are capable of fulfilling the time reproducibility requirement. Only a specialized and expensive computer system can completely fulfill these necessary conditions.
Some previously developed FT/ICR systems involve the use of multiple processors. For example, there is a system described in Lecture Notes in Chemistry (Springer, Berlin, 1982) Vol. 31, p. 365, written by R. J. Doyle, Jr., T. J. Buckley, and J. R. Eyler. In this system a Kim-1 microcomputer was used to perform routine tasks of timing and data processing necessary to obtain ion excitation data from the system. An Apple II microcomputer was then used to carry out further processing and handling of the acquired ion excitation data. In effect, the Kim-1 microcomputer takes the place of the specially dedicated fast control hardware needed in the computer system discussed above.
Since the Kim-1 microcomputer is limited in speed and in memory, it does not have the ability to accomplish all the necessary steps to control and monitor the detailed ion-cyclotron events. External devices are used, including an "excitation oscillator" or waveform generator, and a "waveform recorder" or waveform digitizer.
The Kim-1 acts as a specially dedicated hardware device capable of reproducing the necessary time and phase relationship between ion excitation and data acquisition. Therefore, it can be used together with the waveform generator and the waveform digitizer as a data handling device between the FT/ICR experiment and the Apple II microcomputer. The Apple II microcomputer is then responsible for user interaction, program development, graphics, data storage, and Fourier transformation.
The minicomputer and multiple microcomputer systems described above are capable of carrying out FT/ICR experimentation with accurate results. However, these systems are expensive and require a great deal of hardware in order to effectively test ion samples. A FT/ICR system capable of carrying out accurate FT/ICR experimentation at lesser cost and smaller size would be a great advantage. A cheaper system would make experimentation available to more scientists including those without unlimited resources.